In recent years, special interest has been devoted to the area of high energy density materials for generation of propulsive energy. In general, the quest for new fuels involves properties of matter as well as related energetic processes. Specifically for propellants, the limiting parameters of their effectiveness is stability of the fuel, followed by the amount of energy which is released upon reaction, and the mass which is involved. The best available propellants, such as the liquid hydrogen-oxygen mixture, are limited to specific impulses (I.sub.sp) of 450-480 sec. In order to break through the current efficacy threshold of known fuels, new and practical highly energetic processes involving light atomic elements must be forthcoming. If a new class of high energy chemicals were to increase the I.sub.sp above 1000 sec., this would, for instance, enable a single-stage, airliner-sized vehicle to make a horizontal takeoff, convey a 25,000 lb. Payload to orbit and return for a runway landing.
Obviously, energy containing materials are those having electronic atomic and molecular excited states. However, these systems are not suitable for use as fuels due to their extreme instability, sec. and 10.sup.-14 sec. and 10.sup.-6 sec. Nonetheless, efforts are currently under way for establishing the existence of sufficiently metastable energetic excited states.
On the other hand, as will be described, it has been found that certain molecular species have a metastable ground state with a lifetime on the order of hundreds of minutes and with releasable energy, a fact which makes them eminently practical as a fuel or propellant. These species are those which have dissociating ground state surfaces that have deep energy wells which are caused by special interactions with the first excited surface which characterizes the species. The existence of a real potential energy minimum implies practical energy trapping. Once trapped, this energy can dissipate via tunneling or, occasionally, via small interactions with plunging dissociative states of a different symmetry. Such a ground state potential surface is referred to herein as a volcanic ground state.
By way of background, volcanic ground states are, of course, rarities. Two categories of diatomic or polyatomic systems having volcanic ground states have been analyzed thus far. First, as reported in the Journal of Chemical Physics, Vol. 80, p. 1900, 1984 by C. A. Nicolaides, et al., slices of hypersurfaces of a special class of polyatomic molecules show deep minima and the volcanic form along a reaction coordinate which leads to neutral ground and excited fragments. This situation emerges as a natural consequence of intramolecular charge transfer at very narrow avoided crossings whose geometric dependence is predicted by the maximum ionicity of excited state (MIES) theory. However, calculations on clusters such as (H.sub.2).sub.2 have thus far shown that the minima of these ground hypersurfaces are only virtual, i.e. existing for only one slice through the surface. Also, the chemically bound first excited state formed at the avoided crossing dissociates via non-adiabatic coupling within 10.sup.-13 sec. Thus, in the first category, not only are only virtual volcanic ground states exhibited, the lifetimes of corresponding excited states are much too short for the corresponding molecules to be of practical value for energy storage and release.
On the other hand, in a second category, if a volcanic ground state exists in a diatomic system, there is only one reaction coordinate and thus the minimum is always real. This being the case, at least for cryogenically-stored diatomics, if they exhibit a volcanic ground state and if the corresponding lowest rotation-vibration level has a sufficiently long lifetime, they are candidates for highly energetic practical fuels.
Note that all prior theoretical research on such diatomic and polyatomic systems has been in the chemical bonding area and has involved lifetimes which correspond to transient species observable mass spectroscopically. In this regard, the distinguishing volcanic bonding feature was first determined by Linus Pauling for the case of the He.sub.2.sup.++ 1 .SIGMA..sub.g.sup.+ ground state. As reported in the Journal of Chemical Physics, Vol. 1, p. 56, 1933, Pauling obtained a volcanic potential energy curve which theoretically allowed tunneling and fragmentation to He.sup.+.sub.+ He.sup.+. Pauling explained this property in a valence-bond picture, in terms of covalent-ionic mixing in the wave-function and in particular of the structures He. +He. and He: +He. Furthermore, he predicted that for large R, the potential curve is defined by the 1/R Coulomb repulsion between the two He.sup.+ ions and that at about 1.3.ANG. the resonance interaction of the electrons becomes important, causing the force to become attractive at about 1.1.ANG.. This led Pauling to the postulation of a molecule which would be sufficiently metastable to give rise to a band spectrum. Referring to the intrinsic instability of such a diatomic ground state, Pauling predicted that the four vibrational levels would show pronounced autodissociation characteristics, meaning that they would have short lifetimes. As will be seen, Pauling's qualitative predictions were correct only for the higher vibrational levels and not for the v=0 or even the v=1 levels.
Regardless, Pauling's paper signaled the beginning of a series of publications on the chemical bonding of doubly ionized diatomics. For example, further theoretical work on the bond formation and the potential energy function, V(R), of He.sub.2.sup.++ 1 .SIGMA..sub.g.sup.+ has been published using methods which include electron correlation. A previous study by C. A. Nicolaides, et al., published in the Journal of Chemical Physics, Vol. 114, p 1, 1987, relating to the adiabatic surfaces of the .sup.2 .SIGMA..sub.g.sup.+ Rydberg series of He.sub.2.sup.+ a calculation of the He.sub.2.sup.++ 1 .SIGMA..sub.g.sup.+ threshold. This study also showed how the volcano form is created by the two-electron rearrangement 1.sigma..sub.g 1.sigma..sub.u.sup.2 .rarw..fwdarw.1.sigma..sub.g.sup.2 n.sigma..sub.g. Similarly, this study showed that the configurational mixing which determines the character of the He.sub.2.sup.++ 1 .SIGMA..sub.g.sup.+ state is mainly 1.sigma..sub.g.sup.2 .rarw..fwdarw.1.sigma..sub.u.sup.2 .rarw..fwdarw. 1.sigma..sub.g n.sigma..sub.g. No predictions as to the lifetime of the He.sub.2.sup.++ 1 .SIGMA..sub.g.sup.+ ground state were formed at that time, and no predictions as to the chemistry of He.sub.2.sup.++ were made.
As regards other doubly ionized diatomics with volcanic ground states, as reported in the Canadian Journal of Physics, Vol. 36, p. 1585, 1958, P. K. Carrol observed a spectral emission in N.sub.2.sup.++, while as reported in the Journal of Chemical Physics, Vol. 35, p. 575, 1961, Dorman and Morrison, at A. C. Hurley's suggestion, used the Pauling type of analysis in their discussion on the bond formation of CO.sup.++, N.sub.2.sup.++ and NO.sup.++. Since then, much theoretical and experimental work ha been published on such systems. Apart from methods of ambient temperature preparation and observation, the focus of these efforts has been on the scientific realm of molecular structure and spectroscopy with analysis analogous to those used for other molecules. As reported, the lifetimes of the vibrational levels of the studied diatomics deduced from the mass spectroscopic experiments are in the microsecond (10.sup.-6) range. The computations of reported tunneling widths have revealed that this range corresponds to excited rovibrational levels, as opposed to ground levels. Given the conventional spectoscopic aims of the experimental research, longer lifetimes for lower levels were not studied. Indeed, D. L. Cooper simply refers to the widths of the lower levels as negligible in a report published in Chemical Physics Letters, Vol. 132, p. 377, 1986.
Moreover, except for He.sub.2.sup.++, for the dications whose computed potential energy curves are in the literature, it is estimated that the energy which could be released upon induced fragmentation is in the range of 3-7eV. While it would be useful to harness such energies, even if they became available, their magnitude coupled with the values of the masses of the outgoing fragments, would not lead to an impulse capacity which is competitive with the best available mono- or bi-propellant systems.
Note, the only reported observation of He.sub.2.sup.++ was made by M. Guilhaus et al, in the Journal of Physics, Vol. B17, L605, 1984, in which the He.sub.2.sup.++ was produced at ambient temperature and observed by charge-stripping mass spectroscopy. At that time, Guilhaus, et al., reported that no accurate measurement of lifetime could be made to determine whether the diatomic was stable or unstable. It is interesting to note that Guilhaus, et al., could not have discovered lifetimes of the lowest rotation-vibration level for the ground state because the set-up of their mass spectroscopic experiment was not aimed at the detection and spectroscopic analysis of states with lifetimes on the order of hundreds of minutes.
From the above scientific evidence over the years, it has been deemed unlikely for a molecular system to possess simultaneously those crucial optimal characteristics of energy, stability, mass and releasability for the realization of a practical propellant, or any practical energy storage and release system.